Distortion correction in transmission systems



May 17, 1932. H. w. DUDLEY 1,859,030

DISTORTION CORRECTION IN TRANSMISSION SYSTEMS Filed Jan. 4, 1929 H. l/L/ DUDLEV Patented May 17, 1932 vUNITED L'stier s raies y :di

HOMER W. DUDLEY,.OE TEANECK, .frEnsEY, AsSIGNoR 'ro BELL'JJELEEHONE LAB- ORATORIES, INCORPORATED, or NEW YORK, N. Y., A OORPORATION'OE NEW YORK nISToRTIoN CORRECTION IN TRANSMISSION SYSTEMS l Application filed January-4,1929. serial No. 330,169.

vThis invention relates-to systems for trans mitting wave energy, such as telephone trans'- mission systems and the like, and particularly to improvements in the transmission characteristics of suchl systems.` n, l

In systems for transmittingwaves .comprising a plurality of frequencies, and 'in which transmission apparatus havinganonlinear relation between its inputand output is utilized to produce certain transformations in the transmitted Waves, for example, amplification, such apparatus, due to its non linea-r distorting qualities introduces intothe system Waves of additional frequencies formed by some combination ofthe impressed waves. For example, space discharge amplifying devices intelephone transmission systems when overloaded produce undesired. harmonicswhich limit the-effective power 2O that may be transmitted overthe system ifa given degree of articulation to, be obtained.

An object of the invention is to increase.

the energy-carrying capacity of transmission apparatus in such Systems for a given amount of distortion produced in the transmitted waves by such apparatus.

A related object is to reduce the amount of distortion introduced by transmission apparatus in such systems in'waves transmitted thereby for a given Vamount of energy in said These objects are attained in accordance` with the invention by choosing and relatively to transmit a maximum amountfof energy with minimum distortion. l

A preferred embodiment of the invention is in connection with a telephone repeater circuit of the negative impedance type, such as is disclosed and claimed in the U. S. patent. issued to G. Crisson, No. 1,776,311, on

September 23, 1930. This repeater circuit comprises two v.2l-type vacuum tube repeaters associated with a two-way transmission line through a hybrid coil, one of these repeaters arranged to give a series negative impedance effect being connected to the series winding of the'hybrid coil, andthe other i'epeateiarranged to give a shunt negative impedance cffectbeing connected to the bridge points of the hybrid coil. This repeater circuit operates in such manner that the output currents of the twovrepeaters are impressed on the line so as to reinforce each other in the vdirection vtowards the receiver and to annul .each other in the direction towards the transmitter, thus obtaining amplificationv while minimizing echo effects. A 21-type (two way-one repeater element) repeater may be defined as that'type of balanced bridge repeater lwell-known in the art adapted to be inserted in a transmission circuit,'and in which the two arms of the bridge, each comprising one'section ofthe transmission circuit, are balanced against each other so that one repeater element may bey employed for repeating currents between the sections in opposite directionsy without setting up a singing condition.A In its usual form, the .Q1-type repeater comprises a three windingor hybrid coil transformer having two of its windings connected in series between the two sections of the transmission circuit and the third winding inductively relatedto the other 'two' windings, and a one way amplifier circuit having its output terminals connected to said thirdwinding and its input terminals con'- nected across the transmission circuit at the junction points between the other two windings of the transformer. y

Without affecting the desirable result obtained by the combination repeater circuit described inthe above mentioned Crisson patent, addition al advantages may be attainedtherein 'by a modification in accordance with the present invention. These advantages, which" include the increasing of the energycarrying capacity of this type of repeater for a givenv quality of transmission, or a reduction in the amount o f distortion produced in the transmitted waves by the repeater for a given load, are attained by proper selection of the disthe fundamental and the odd harmonics generated by the two tubes are transmitted over the line in one direction only-for example, towards the listener. As regards that one direction, however, the even harmonics generated by the two amplifier tubes neutralize each other in the line, thus improving articulation in the system. Some of the energy of the even harmonics will be returned over the line in the direction of incoming waves, but the echoes produced thereby will not seriously affect the talker as these even harmonics do not contain a great deal o f intelligibility. In the general case where the voltage wave applied` to the inputs of the two repeaters contains several frequencies instead of single harmonics of the impressed frequencies, it is the set of sum and diiference frequencies, including the even harmonicsv of all the impressed frequencies, that is suppressed for one direction .ofv transmission In another applicationl of the invention to the same repeater circuit, the distortion producing apparatus is so chosen asto have sub.- stantially complementary characteristics. In this case the opposite poling of the two 2i-type repeaters will result in the elimina tion of all combination frequencies includ.-

ing the even and odd harmonics of the im.- pressed waves, in any desired branch of the system.

The objects and advantages of the invenT tionv will be brought out more clearly in the following detailed description when read in Connection with the accompanying drawings in which Figs. 1 and 2 show sets of curves illustrating the theory on which the invention is based and Fig, 3 shows an application of the invention toa two-way repeater of the negative impedance type.

In Fig. 1, the solid curve Q1 indicates the non-lineal' relation existing between the output and input of a given system producing distortion in the form of harmonics, and the solid curve C2 the non-.linear relation existing between the output and input of a simi lar' distortionproduoing system, as may be obtained, for example, when the distortion devices in the two systems are identical vacuum tubes r other devices having sub-V stantially parabolic characteristics. If the outputs of the two systems are of the proper v magnitude and are superposed in the proper phase relation the two systems together may be made to have a combined input-output characteristic, such as indicated by the dotdash curve G3, which is symmetrical about a certain point, the effect being that the even harmonics generated by the two systems annul each other. This may be best explained by the following mathematical analysis.

In the .case of a vacuum tube the 'output current 2f is related to the input voltage e by the quation:

where E0 represents the biasing direct cur-l rent voltage applied to the grid of each tube.

for example, by means of a grid battery, and

th-e terms -l-El sin w15 and -E1 sin wt represent A. C. waves applied to the inputs of the respective tubes 180 out of phase; that is, if they impressed waves are sinusoidal, when one tube has its grid' at the maximum positive A. C. potential, the other tube has its grid at the maximum negativel A. C. potential.

The output currents il and '2 of the two tubes may be represented as:

i1=fa+wiE #e152 sin et+ a2 (E02 QEOEl sin wt -l-E12 sin2 wt) and f/'TfaJfaiErctEi sin et+ (Z2 (-42102 vSlll @d-E12 Sing' (at respectively.

In general, if the output currents l and 2 are fed into a common circuit, all or any part of these currents may be made to add or subtract in a branch of the common circuit. If the circuits 4are arranged so that the two currents add in the common circuit, the resultant current obtained therein may be expressed as the iirst term of this equation representing` expressed as made to feed back towards t-he talker rather than towards thel listener.

If the circuits are arranged so that the two currents subtract in the. common circuit, the resultant current obtained therein may be the single term of this equation representing the amplified fundamental current. As this current ldoes possess intelligibility it should be made to flow towards `the listener rather than towards the talker.

In Fig. 2 the solid curve C1 indicates the non-linear relation existing between the output and input of a given system producing distortion in the form ofharmonics and the i, fig-:2151 (ali-Qaf/QEO) sin et solid curve C2 represents the non-linear relation existing between output and input of a systemin which the distortionis of'a different type. If these two systems have distortion characteristics which are substantially complementary, as indicated by the shapes given to curves C1 and C2 of Fig. 2, and if their outputs are of the proper magnitude and are superposed in a system in proper phase relationthe two systems taken together will have a linear input-output lcharacteristic such as vindicated by the dot-dash line C3, the effect being that all combination frequencies including both the even and odd harmonics will be balanced out. This may be explained by the following mathematical analysis:

Let it be assumed that a signaling voltage e is respectively applied to the inputs of two pieces of apparatus, such as space discharge devices, having non-linear input-output characteristics which are effectively complementary. The average output current of the two tubes will then be given by an equation where a0 and al are constants of the tubes and the associated circuits.

The output current of the two tubes will then be given by the Equations 9' and 10, respectively.

0: ao ale add in the common circuit the resulting current obtained therein may beV expressed as The first term on the rightof this equation represents a D. C. component, the second a wave which is a reproduction ofthe original wave applied to the inputs of the distortion producing apparatus except that itisv on a dili'erent scale, all distortion components lproduced by the two tubes having been effectively canceled out in the common circuit by combining the two output waves therein vin this manner. I

If thecircuits are arranged so that the output currents oppose each other in the common circuit, the resultant current obtained therein may be expressed as v The terms of Equation (l2) represent the combination waves due to the non-linear distortionv apparatus, which are not canceled out.

the same time the line is maintained conductively continuous through each repeater rendering unnecessary the use of composite sets when the line is used also for transmitting direct current or low frequently alternating currents, as in telegraphy. In order to bring out clearly the particular modifications which are made in the repeater circuit of the Crisson patent in accordance with the present invention, the details of the Crisson repeater illustrated in Fig. 3 and its manner of operation will be first described. Referring to Fig. 3 it will be seen that the repeater circuit illustrated includes a hybrid coil II1 having the usual line windings and series winding, which is connected between the two parts LW and LE of the transmission line extending between two stations designated as the west station and the east station, respectively.

To the series winding of hybrid coil I-Il is connected a 21-type repeater circuit X, and also a transformer T3 and a network N1 for balancing the hybrid coil H1 'and the two parts of the associated line LE and LW to prevent the repeater X from singing.

The 21V-type repeater'X comprises a-hybrid coil H2 of the usual type connected between the terminals ofthe series winding of hybrid coil H1 and the terminals of the primary winding of transformer T2, andan amplifier A1 having its input circuit connected to the bridge points between the line windings of hybrid coil H2 through an input transformer T5, and its output circuit connected through other'apparatus represented by the box B1 to the series winding of hybrid coil H2. The apparatus in box `Bl may, comprise Vfor example, an output transformer, a transmission equalizer if it is necessary, a filter and any other apparatus needed.

To the bridge points between the line windings of hybrid coil H1 is connected a circuit including the condensers C2 and C3, a C3d-type repeater Y, a condenser C1, a resistance R1, and a balancing network N2. The condensers C1, resistance R1 and network N are provided for balancing the condensers C2 and C3, the resistance of the line windings of hybrid coil Hl and the two parts of the transmission line LE and LW, respectively, so that the 2ltype repeater Y will not sing. The condensers'C2 and C2 serve to prevent the passage of direct current or low frequency alternating current to the 2l-type repeater Y, and also, if given proper values, to equalize phase shifts introduced by the hybrid coil` H1.

The Ql-type repeater Y comprises a hybrid coil H3, which is connected between the network N2 and the bridge points of hybrid coil H2, and an amplifier A2 having its input connected across the bridge points between the line windings of hybrid coil H3 through an input transformer T5 and its output connected to the series winding of hybrid coil H2 through apparatus represented by the box B2, which apparatus may be similar to that included in box Bl of repeater X described above.

In the manner explained in detail in the above mentioned Crisson patent, .theQl-type repeater X is connected with respect to the two parts of the line LE and LW through hybrid coil H1 in a manner which has been designated as the direct connection, that is, a connection such that for transmission in the system in either direction the input currents to the repeater X and the resultant amplified output currents from the repeater X flow in the line wires LE and LW in the same direction. The repeater circuit Y, on the other hand, is connected with respect to the two parts of the transmission line LE and LW through hybrid coil H1 in the manner which has been designated as the reversed connection, that is, a connection so that for transmission over thesystem in either direction the input currents to the repeater Y and the amplified output currents from the repeater Y flow in one part of the transmission circuit in opposite directions and in the other part of the transmission circuit in the same direction.

For example, if currents are being received over line LW in the direction indicated by the light straight arrows, the connection of repeater X to hybrid coil H1 is such that the resulting amplified current from repeater X fiowsin the two parts of the line LW and LE in the same direction, as indicated by the heavy straight arrows. The connection of the repeater Y with respect to the hybrid coil H2, however, is such that for currents received overL line LW in the direction of the light straight arrows, the resulting amplified currents from repeater Y flow in the portion of the line LW in the direction opposite to that of the received currents and in the portion of line LE flow in the same direction asthe received currents, as indicated by the wavy arrows. In this way the complete repeater comprising the two l-type repeaters X and Y operates so that for transmission over the line in either direction an amplified wave will always be transmitted to the listener, while the waves tending to travel toward the speaker will be minimized. By properly adjusting the gains of the two 21-type repeater circuits, of course, the waves tending to travel toward the speaker may be completely neutralized so that no echoes are received by him.

Now it is well known that vacuum tube amplifiers, such as are used in the amplifiers A1 and A2, inV repeaters X and Y, respectively, because of their non-linear distorting qualities, when overloaded, will tend to generate waves of frequencies which are some combination. of the frequencies of the waves impressed on their input circuitsf In the system of Fig. 3, these combination waves, or distortion waves produced by the amplifiers A1 and A2 when overloaded, when transmitted into the main transmission line will interfere with the speech waves therein and impair articulation.

It is a purpose of the present invention to modify the repeater circuit of the Crisson patent, which has just been described, so that without affecting appreciably its advantageous operation as regards echo elimination, the effect of at least a portion of the distortion waves may be neutralized. This is accomplished in accordance with the invention by relatively arranging the circuits of the two repeaters X and Y, or the connections thereof to the transmission circuit, so that at least portions of the distortion currents generated by the two repeaters fiow in one part of the main transmission circuit in opposite directions. This will enable the energy level or power in the system to be increased without increasing the size of the tubes of amplifiers A1 or A2. In other words, it will enable the energy carrying capacity of given amplifier tubes and the repeater a whole to be increased without appreciably impairing the quality of transmission in the system. A simple method of obtaining this result is by controlling the relative poling of the input circuits of the tubes in amplifiers A1 and A2.

ver'

i both switches 21e the input voltage Waves will be impressed circuits 'of the repeater X, ora switch S1'in the' connections between repeater X yandrhybrid coil H1. 'Oef course, the esame result vcould be obtained byv means fof similar 'switches inthe 2l-type repeater Y and in the connections of repeater Y to hybrid coil-H1.

Itis obvious that any change in the'connection ofthe amplifier tubes A1' andA2 for the purpose of applying 4a portion of the distortion waves produced thereby to the main transmission circuit in opposite phases, which will also change the relative connection of the two amplifiers A1 and A2 to the hybrid coil H1 would destroy the operation of the repeater circuit as a whole as regards elimination of echo currents.

If the connections of the two amplifier circuits X and Y are not normally such that the distortion waves or a portion thereof produced by the two amplifier tubes A1 and A2 are applied' to a portion of the transmission circuit in opposite phases, it is, of course, possible to attain this condition by reversing the double pole switch S2 to reverse the polarity of the grid of amplifier tube A2. The ef feet of this however, would beto change repeater Y from its reversed connection to a direct connection, such as that of repeater X, and thus to defeat the main purpose of the repeater as a whole. If, however, both switches S2 and S3 are reversed together, the only effective change which will occur in the operation of the circuit as a whole will be that the opposite half of the wave is impressed on the rid of tube A2. By reversing S2 and S3 together, therefore,

on the grid of ampliiier tube A2 in repeater Y and the grid amplifier tube A1 in repeater X in opposite phase, that is, one grid will be at a maximum positive potential when the other grid is at maximum negative potential. This will .insure that, as has been explained in connection with Figs. 1 and 2, at least a portion of the distortion waves produced by the two tubes will be impressed on the transmission circuit in opposite phase, but will not change the net effect of the repeater circuit as to the direction of the amplified speech waves impressed on the transmission circuit with respect to the direction of the input waves therein.

The same eiiect as regards the elimination of a portion of the distortion currents as is obtained with the double current reversal in repeater circuit Y by the simultaneous reversal of switches S2 and S3, may be obtained by reversing the single double pole switch S1 located between the repeater Y and the hybrid coil H1, or by reversing a similar switch located between the repeater X and the hybrid coil H1. Reversal of the switch vS1 will change the polarity of the grid of amplifier tube A2 but will not change the relation of repeater Y to hybrid coil H1 as regards the relative direction of the input and amplified speech waves in the transmission circuit, for, the waves traverse 'the circuit including the switch S1 twice (as input and output currents), and the net effect as to the main speech currents is the same as if fz-5 S1 in the manner pointed out to properly pole the repeaters X and Y with respect to the mam transmission circuit, will depend Vupon'the choice of tubes for use therein. If

the tubes selected for the two amplifiers A1 and A2 have complementary distortion `ch'a'racteristics, as explained above in connection with the description of Fig. 2 all distortion components including both even and odd` harmonics of the impressed waves may be veliminated in the main transmission circuit as regards both talker and listener. If, on '450 the other hand, the Itubes selected have substantially identical characteristics, as explained in 'connection with the description of F ig. l the even harmonics of the impressed y wave, which are the' most troublesome, may be eliminated in the ymaintransmission circuit as regards one direction of transmission, for example, in the direction toward Ythe listener, thus improving articulation in the system.V

In the important frequency range'in lthe repeating system which has been described,

itis desirable from a transmission standpoint that Vthe phases of the amplified currents transmitted from the two tubes to the main transmission circuit, be the same for the odd harmonics and 180 apart forthe even harmonics. Thus, 'if apparatus with appreciable phase shift be used in repeater X, for ex- 1 1 ample, in the apparatus which has been indicated by the box B1 therein, similar circuit apparatus rshould be included in repeater Y in the box B2.

Although the invention has been illustrated and described in connection with a telephone and'input, or to any other system or apparatus within the scope of the appended claims. What is claimed is: l. Ina transmission system,-a transmission line for transmitting electrical waves, tWoor more two-way repeaters for amplifying said waves, each of said repeaters having "a n onlinear amplification characteristic-'tending to lIl() produce Wave distortion in the system, said repeaters being so connected with said line that in a portion thereof the distortion energy produced by one of said repeaters opposes that produced by another of said repeaters.

2. In a transmission system, a transmission line for transmitting electrical waves, two or more transmission devices connected effectively in said line and in substantially conjugate relation with each other, each of said devices having a noii-linear output vs. input .characteristic tending to produce wave distortion in the system, said devices being relasaid repeaters having a non-linear output-inut transmission characteristic, said repeaters eing so connected with. said line that in a portion thereof the distortion energy produced by one of said repeaters opposes that produced by another of said repeaters.

4. In a transmission system, a transmis- 4sion circuit for transmitting waves comprising a plurality of frequencies, two or more Atransmission devices connected effectively in .said line and in conjugate relation` to each other, each of said devices having al non-linear output vs. inputY characteristic, the method i of improving the transmission qualities of the system which consists in relatively poling said transmission devices with reference to the applied waves so that a portion of the distortion energy introduced into a certain part of said system by the respective transmission devices effectively cancels another portion of vsaid distortion energy. 45

5. In combination, a line for transmitting waves of a plurality of frequencies, a circuit for amplifying said waves connected between sections of said line, said circuit comprising two or more amplifying devices, having a non-linear relation between input and output, said amplifying devices being so connected with said line that the repeated waves produced in one section thereof by one of said devices are in the same direction as the repeated waves in said one line section produced by the other of said devices, and the repeated waves produced in the other section of line by said one amplifying device opposes the repeated waves in said other section of line produced by said other of said devices, said of the harmonic frequencies of the waves applied thereto generated by said one amplifyingdevice opposes in said one line section 

